Detecting ADV7181C locked status

Does anyone know of a way to determine whether or not the video input is locked by analyzing the output signals, VS, HS, etc.? The chip will be configured in composite, component or s-video modes with the output as RGB 4:4:4. I'd like to detect input status without having to read the status register via I2C. The outputs will be processed by an FPGA, so there is an opportunity for some clever logic to analyze them.